For varying the load torques and the speed of a driven system, in particular of a system for driving a vehicle, it is desired to operate the combustion engine under nearly constant conditions, wherein energy not transmitted to the wheels during a steady operation of the vehicle is received in an accumulator which supplies energy for an increased demand when the vehicle is to be accelerated. The accumulator is additionally used to receive energy which may be recovered in braking the vehicle.
According to a conventional system, the combustion engine is coupled to a free-wheel accumulating energy generated by the combustion engine under optimum conditions of operation and supplying energy to the vehicle through a hydrostatic transmission when desired. Energy recovered in braking may be supplied to the free-wheel as well Such system suffers from substantial drawbacks, for example, the free-wheel losses are highest with maximum speed and when the vehicle is stopped.
Another conventional transmission ("Olhydraulik und Pneumatik", 1980 No. 7, page 525) comprises a power branching system, wherein a hydrostatic transmission to optimize the acceleration and deceleration of the vehicle and a mechanical transmission in parallel are provided to optimize the gear transmission efficiency at steady speed. The hydrostatic transmission comprises a clutch each for the input and output., while the mechanical transmission comprises a further clutch. The particular ratio desired may be selected by actuating the clutches. Furthermore, the machines of the hydrostatic transmission are adjustable in throughput to additionally vary the acceleration and deceleration.